The present invention relates to a rotatable anvil for a rotary cutting unit, comprising an axle, at least one anvil portion adapted to cooperate with a knife member of a rotary cutter and a pair of load transmitting portions adapted to abut a pair of abutment members of said rotary cutter, said pair of load transmitting portions being arranged on each side of said anvil portion.
The invention also relates to a rotary cutting unit comprising a rotary cutter and such a rotatable anvil.
Such a rotary cutting unit is known from U.S. Pat. No. B-6,244,148, and comprises a rotary cutter in working relationship with a rotary anvil. The rotary cutter is provided with a substantially circular-cylindrical body having a cylindrical cutting surface and at least one knife member protruding from said surface, the radially peripheral part of said knife member having a diameter larger than that of said surface. Each side of the rotary cutter is provided with an axle supported in bearings. Between the axles and the surface, i.e. on each side of said surface, a pair of annular abutment members are provided. The abutment members have a diameter larger than that of said surface, in order to allow abutment against a pair of load receiving portions of the anvil.
The anvil is provided with an anvil portion and said pair of load receiving portions. The anvil portion is adapted to cooperate with the knife member of the rotary cutting drum, whereas the load receiving portions are adapted to abut said abutment members of the rotary cutter. The anvil is supported in bearings outside the anvil portion and outside the load bearing portions, as seen in the axial extension of the anvil.
Furthermore, the abutment members have a diameter which is substantially the same as the radially peripheral part of said knife member. The abutment members are adapted to lie against and transmit load such that a predetermined pressure is exerted on the load receiving portions of the anvil to achieve a desired cutting property. Optionally, the abutment may also transmit rotation of the rotary cutter drum to said anvil surface, such that it turns in a direction opposite to that of said rotary cutter.
A product is cut from a web introduced between the drums by the centrally arranged knife member.
The described rotary cutter however suffers from the drawback that the portions of the knife members in the axial center of the rotary cutting drum do not cut as precisely as the portions of the knife members closer to the axial periphery thereof. This is due to the fact that the rotary cutting drum exerts a pressure onto the anvil surface via the abutment members, thereby causing the anvil to be bent. This is shown in FIG. 6, illustrating the principle of a prior art anvil A corresponding to the anvil disclosed in U.S. Pat. No. A-6,244,148. A pair of load receiving portions B are arranged on respective sides of the axial extension of an anvil portion C, whereas a pair of annular support surfaces D for cooperating with bearings are arranged on either sides of the pair of load receiving portions B. When load E is applied to the load receiving portions, the central portion of the anvil A will be bent slightly downwards as shown (exaggerated) by the broken line F due to the counter directed force G on the surface D, i.e. at the bearings. Such bending may be denoted negative bending.
Another prior art rotary cutting unit has an anvil for cooperation with a rotary cutting drum having two or more knife members arranged side by side. Such a rotary cutter not only suffers from the drawback described above, but also in that it has a long axial extension, causing the anvil drum and the rotary cutting drum to also be bent by gravity, i.e. the longer and heavier the anvil, the more it will be bent negatively by gravity that will add to the described effect.
The use of two or three parallel knife members or one large knife member on a long rotary cutter to cooperate with such an anvil will thus only have acceptable cutting properties at the peripheral portion of the anvil, whereas the knife member closer to the central portions may not cut through the web to be cut.